In acoustic well logging, it is customary to measure the compressional wave velocity of earth formations surrounding boreholes. A conventional compressional wave velocity logging system includes a cylindrical logging sonde suitable for suspension downhole in a borehole liquid, a source connected to the sonde for generating compressional waves in the borehole liquid, and one or more detectors connected to the sonde and spaced apart from the compressional wave source for detecting compressional waves in the borehole liquid. A compressional wave in the borehole liquid generated by the source is refracted into the earth formation surrounding the borehole. It propagates through a portion of the formation and is refracted back into the borehole liquid at a point adjacent to the detector and is then detected by the detector. The ratio of the distance between the source and detector to the time between generation and detection of the compressional wave yields the compressional wave velocity of the formation. Information important for production of oil and gas from subterranean earth formations may be derived from the compressional wave velocities of such formations.
When a compressional wave generated by a compressional wave source in the borehole liquid reaches the borehole wall, it produces a refracted compressional wave in the surrounding earth formation as described above. In addition, it also produces a refracted shear wave in the surrounding earth formation, and a guided wave which travels in the borehole liquid and part of the formation adjacent to the borehole. In hard formations where the shear wave velocities of the formations are greater than the velocity of sound in the borehole liquid, part of such shear wave is refracted back into the borehole liquid in the form of a compressional wave and reaches the detector in the logging sonde. The guided wave is also detected by such detector. Any wave that is one of the three types of waves detected by the detector may be called an arrival: the compressional waves in the borehole liquid caused by refraction of compressional waves in the formation the compressional wave arrivals, those caused by refraction of shear waves in the formation the shear wave arrivals, and those caused by guided waves the guided wave arrivals. Thus, the signal detected by the detector is a composite signal which includes the compressional wave arrival, the shear wave arrival and the guided wave arrival. Compressional waves travel faster than shear waves and shear waves usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compressional wave arrival is the first arrival, the shear wave arrival the second arrival, and the guided wave arrival the last arrival.
It is well known that shear wave velocity logging may also yield information important for production of oil and gas from subterranean earth formations. The ratio between the shear wave velocity and compressional wave velocity may reveal the rock lithology of the subterranean earth formations. The shear wave velocity log may also enable seismic shear wave time sections to be converted into depth sections. The shear wave log is useful in determining other important characteristics of earth formations such as shear stress, porosity, fluid saturation and the presence of fractures. The shear wave log may also be helpful for determining the stress state around the borehole which is very important in designing hydraulic fracture treatments.
Asymmetric compressional wave sources have been developed for logging shear wave velocity. Using such sources, the amplitude of the shear wave arrival may be significantly higher than that of the compressional wave arrival. By adjusting the triggering level of the detecting and recording systems to discriminate against the compressional wave arrival, the shear wave arrival is detected as the first arrival. It may thus be possible to determine the travel time of shear waves in the formation and therefore the shear wave velocity. Asymmetric sources are disclosed by Angona et al, European patent application No. 31989 and White, U.S. Pat. No. 3,593,255.
In soft formations, such as near surface formations or the Gulf Coast soft shale, the shear wave velocities of such formations are frequently less than the velocity of sound in the borehole liquid.
According to Snell's Law, where the shear wave velocity of the formation is less than the sound velocity in the borehole liquid, the shear waves refracted into the formation will travel away from the borehole, and will not be refracted back into the borehole liquid to reach the detector. Angona et al and White have not disclosed how shear wave velocities may be logged in such circumstances.
Kitsunezaki in U.S. Pat. No. 4,207,961 discloses a device for logging the shear wave velocity of a soft formation. Coils are mounted on a bobbin assembly which is then placed in the magnetic field of a permanent magnet. A current pulse is passed through the coils to drive the bobbin assembly. The movement of the bobbin assembly ejects a volume of water in one direction and simultaneously sucks an equivalent volume of water from the opposite direction. Through the medium of water, the movement of the bobbin indirectly pushes a portion of the borehole wall and pulls another portion on the other side of the bobbin assembly. Such excitation imparted to the borehole wall will generate shear waves in soft formations which are detected at points in the borehole liquid vertically spaced from the bobbin assembly.
The U.S. Pat. No. 4,207,961 to Kitsunezaki has been assigned to OYO Corporation of Tokyo, Japan. In an OYO Technical Note dated November 1980 and entitled, "Development of the Suspension S-wave Logging System," by Ogura, Nakanishi and Morita, a solenoid type electromagnetic excitor for generating shear waves is disclosed which appears to be the same as the device disclosed in U.S. Pat. No. 4,207,961. In the Note, it is stated that the solenoid type excitor shear wave logging system can be used to measure shear wave velocites up to speeds of around 1 km/sec. or 3000 ft/sec. Thus it appears that the device disclosed by Kitsunezaki may not be capable of logging shear wave velocities above 3000 ft/sec. The Note further states that data from experiments using such system show that the observed shear wave amplitude falls off dramatically with increases in shear wave velocity of the formation, and that at shear wave velocities as low as 450 m/sec. or about 1350 ft/sec., the observed shear wave amplitude become extremely small. Hence it may even be difficult to use such device to log shear wave velocities between 1350 ft/sec. and 3000 ft/sec. In the article "A New Method for Shear-Wave Logging", Geophysics Vol. 45, No. 10 (October 1980) pp. 1489-1506, Kitsunezaki described a logging device which appears to be the same as the device of U.S. Pat. No. 4,207,961. Kitsunezaki, on page 1500 of the article, stated that the driving mechanism of such logging device has problems in logging formations with higher shear wave velocities.
It also appears that Kitsunezaki's device must be stationary while it is being used for generating shear waves in earth formations in the manner described above. This requirement will slow down the logging process. lt will also increase the likelihood of the logging device being trapped in the well and the likelihood of losing the device.